I would appreciate some insight into a couple of things I'm considering.
L Jet vs Motronic Intake Manifold:
I converted my 3.5 from carb to EFI last winter using the intake manifold from an 81 528i. Everything went well. Since then I have picked up an intake from an 89 735i and a Tii brake booster. I've been told that using the equal length of the motronic intake runners versus the quite substantially unequal lengths of the L Jet runners (but larger diameter) will have advantages in power. Does this seem reasonable?
Strokers:
Can someone explain what this term means? I think that this is increasing the displacement by changing the stroke not the bore. How?
Thanks
Intake design is a very complex and multifaceted problem. The largest problem is designing an intake with a high efficiency at large RPM ranges; the best intakes are designed to operate in a ~1K RPM band, rather than the ~5K RPM band you use on a daily basis.
In general, equal-length runners are good because it evens out the air flow to each cylinder. Then there's the issue of runner design, which you hit upon. Longer runners typically improve the low-end flow, but restrict the top end, partially due to laminar and turbulent flow effects. This is why most suggest using the E21 320i intake runners with the L-Jet manifold; they are longer, which gives better low-end torque response.
The second part of an intake is the plenum. The plenum is the volume after the throttle but before the runners in our cars. A larger plenum volume helps even out the pressure going to each runner, but also acts as more of a drag at higher RPMs depending on how the intake to the plenum is set up.
The best manifolds also include internal structures called velocity stacks. These are special inlets (usually positioned in the plenum acting as the runner inlet) designed to improve the flow of air into the plenum by altering the boundary and flow characteristics at the inlet. In most race cars, they look like tiny trumpets. They have been a standard feature in BMW M-cars since the start.
So, the net is that the 3.5 Motronic intake performs better than the stock L-Jet intake. It is very possible to make a better intake, however. Most of the Alpha-N guys have a great manifold based on the old Gr. 2 / 5 racing engines.
Stroking, as you mentioned, is specifically increasing the stroke without altering the bore. If both are changed, the motor is "bored and stroked". Both techniques are means to increase the displacement. The stroker uses a modified crankshaft to increase the stroke. In our case, the E3 3.3Li crankshaft would be a "stroker" part, as the crankshaft has a longer stroke than the original B34/35 crankshaft. The S38B38 crankshaft also works, but is decidedly expensive. The downside to stroking is the increase in forces on the piston. The piston has farther to go in the same amount of time, which means the piston linear velocity is higher, which places more stress on the various fasteners used in the piston, such as the wrist pins and bolts linking the two connecting rod sections.
Boring and stroking are used to provide different engine responses in addition to increasing the displacement. For example, the M90 feels very different from the M30B35, even though they are almost the same engine aside from the bore and stroke configuration. The M90 is an oversquare engine (bore>stroke) and makes excellent horsepower, even at high RPMs. The M30B35 is almost a square engine (bore=stroke) so each revolution makes more torque, as the engine is able to use more of the combustion pressure to push the pistons. (the pressure imparts a higher impulse to the piston since it is imparting the force over a longer distance) The tradeoff is the M30B35 makes less horsepower at high RPMs due to the longer stroke. (the combustion pressure has less time to impart enough impulse to the piston due to the high RPM, so the pressure on the piston is no longer approximately constant over the whole stroke as it is at lower RPMs) Having a bigger bore means the combustion pressure is spread over a wider area, giving more torque at high RPMs.
